12 years old is a great age to start weightlifting. Most kids are old enough by that time to have a long enough attention span to complete a workout that lasts 45 minutes to an hour, and enough motor control to do the movements required in the sport safely.
Most 12 year olds should still not train more than 3 days a week though. A Lot of parents and coaches are very tempted to push kids to train more often that that, but if you do you run the risk of them burning out before they ever reach the age when they should be performing at their best. Because the goal should be to develop future Olympians, pushing kids into 5 or 6 workouts per week then watching them burn out before age 18 is not a great strategy.
But 12 years old is a delicate age. Successfully coaching this age group requires keeping the interest and motivation high, while also keeping them from burning out. Keeping then training only 3 days a week goes a long way toward keeping burn outs from happening. But many kids get really, really bored doing workouts which require sticking to a certain percentage of their maximum snatch or clean and jerk. Kids love to lift heavy, and they love to max out. Unfortunately, constantly doing nothing but maxing out is not really conducing to their long term development in the sport. One way to get around this is to let them lift as heavy as they want but using a plan where most workouts contain complexes containing 3 to 5 total reps.
I have had really good luck allowing kids to go as heavy as they can, but holding the weight down by making sure that most of the exercises they do were complexes involving a total of 2 to 5 reps. Things like 2 snatch pulls + a snatch + and OHS, or a snatch + a snatch grip push press + an OHS both work great for the snatch. A clean pull + a clean + 2 front squats, or just 2 clean pulls + 1 clean and jerk work great for the clean and jerk. Using complexes like this allows the coach to let the kids train right up to their absolute maximum ability most days but still make sure that most days they cannot go beyond 90% of their best snatch or clean and jerk. Personally I have had good luck using complexes on Mondays that require a total of 4 to 5 reps in one complex, complexes which require 2 to 3 lifts per complex on Wednesdays and then complexes with only 1 to 2 lifts on Fridays.
Working up to a maximum on a snatch complex like this, then a maximum on a clean and jerk complex, then 3 to 5 sets of front squats or back squats is a workout t hat takes most kids in the middle school age group about 45 minutes to 1 hour. It gets results, and because the kids are always pushing for a new maximum on whatever complex we are doing that day, they never seem to get bored. If you are working with this age group, give this type of workout a try!